August 24, 2000
12:00 AM (EDT)

News Release Number: STScI-2000-29

Hubble Takes Census of Elusive Brown Dwarf Stars

August 24, 2000: Astronomers using NASA's Hubble Space Telescope have carried out the most complete
inventory to date of brown dwarfs, one of the universe's most elusive types of
objects, which dwell in limbo between stars and planets. The Hubble census provides
new and compelling evidence that stars and planets form in different ways. Because
the brown dwarfs "bridge the gap" between stars and planets, their properties
reveal new and unique insights into how stars and planets form.

Q & A: Understanding the Discovery

1.
What are brown dwarfs?

Considered an astronomical oddity only a few years ago, brown dwarfs are intriguing objects that, unlike stars, are too low in mass to burn hydrogen, but are more massive than planets. At 15 to 80 times the mass of Jupiter, the light they do emit is so faint that it has been hard to tell how many of them are scattered throughout the galaxy, and whether their formation process is similar to that of stars, planets, or neither of these. Only a few years ago astronomers commonly believed that brown dwarfs are rare, perhaps because the process that makes stars "stops working" at lower masses. The Hubble results provide evidence that this isn't the case.

2.
What are the results of the census?

The Hubble finding shows stars and brown dwarfs form in the same way. Like stars, low-mass brown dwarfs are more plentiful than heavyweights. This trend continues down to very low, almost planetary masses. The Hubble results also offer the strongest evidence to date that brown dwarfs are a completely different population from the recently discovered planets that orbit nearby stars. The census showed that brown dwarfs are, remarkably, far more common in isolation than in orbit around other stars. This finding suggests that the planets, whether in our solar system or beyond, formed very differently than the Sun and other stars. These results support the idea that stars form through the gravitational collapse of cold, dense clouds of gas, whereas planets grow through the buildup of tiny particles of stardust.